Display screen or portion thereof with a graphical user interface



FIG. 1 is a front view of a display screen or portion thereof with a graphical user interface showing the new design according to a first embodiment; and,

FIG. 2 is a front view of the display screen or portion thereof with the graphical user interface showing the new design according to a second embodiment.

Within the drawings, the dashed lines, and the areas within the dashed lines, show the display screen or portion thereof and portions of the graphical user interface that form no part of the claimed design. 

CLAIM The ornamental design for a display screen or portion thereof with a graphical user interface, as shown and described. 